Many processes and devices have been used for measuring flow rate in different applications. A miniature MEMS based pressure sensor can be used to measure very low flow rates and with a reliable accuracy. Such MEMS based pressure sensors have been implemented, for example, in various flow sensing devices, such as medical applications, some of which utilize silicon piezoresistive sensing technology for measuring very low pressures. Other flow sensing implementations, for example, include environmental applications.
MEMS involve the integration of micro-mechanical elements, sensor actuators, and electronic components on a common silicon substrate through the use of microfabrication technology. While the electronics can be fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components can be fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. MEMS promises to revolutionize nearly every product category by bringing together silicon based microelectronics with micromachining technology, making possible the realization of complete “system-on-a-chip ”. MEMS is an enabling technology allowing the development of “smart ” products, while augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators, while expanding the space of possible designs and applications.
The majority of prior art MEMS pressure transducers produced for the automotive market, for example, typically include a four-resistor Wheatstone bridge fabricated on a single monolithic die using bulk-etch micromachining technology. The piezoresistive elements integrated into the sensor die can be located along the periphery of the pressure sensing diaphragm at points appropriate for strain measurement. Such sensors are inexpensive to produce and can be processed in association with integrated circuits on a wafer that may contain a few hundred to a few thousand sensing elements.
In a bridge configuration, the resistance of diagonally opposed legs varies equally and in the same direction, as a function of the mechanical deformation caused by pressure. As the resistance of one set of diagonally opposed legs increases under pressure, the resistance of the other set decreases, and vice versa. Bridge excitation in the form of voltage or current is applied across two opposite corners of the bridge. Any change in voltage (e.g., pressure) can be detected as a voltage difference across the other two corners of the bridge, typically referred to as signal output. Unfortunately for silicon piezoresistive sensors, this voltage difference is quite small. Thus, the sensor must be compensated before it can be used.
Bulk-micromachined silicon pressure sensors typically incorporate the use of a diaphragm that deflects when subjected to a pressure load, and also include a piezoresistive transducer that translates strain to a differential voltage. Metal pads can be used to interface with other system components. Signal-conditioning circuitry for calibration or amplification is optional, but is often included as well. The piezoresistive transducer is strategically placed near the edge of the diaphragm, since that is a high-strain location and each sensor is designed, such that its output voltage is linearly proportional to the applied pressure in its operating range.
In some applications, it is preferred that a signal conditioning/signal amplification capability be incorporated into the sensors. It is believed that there is currently no amplified flow through sensors based on piezoresistive sensing technology in an integrated package. It is further believed that if such a sensor could be implemented, this would help in lowering installation and development costs, while eliminating secondary operations and shortening the design cycle time.